“Sex feeds evolution”

I went yesterday to the Natural History Museum (NHM) in London to see their new Sexual Nature exhibition and it stirred the first motivations and greatly inspired me to write this blog entry.

The exhibition will be on till Oct 2nd 2011 but I would strongly recommend anyone who has the chance to visit it to do it asap. It is quite interesting and very well done for anyone from any age. Although it’s said to be aimed at adults and children over the age of 16, I honestly didn’t see any bits that would shock the faintest of hearts (must even say I was expecting something spicier…). The exhibition is full with interesting facts and curiosities (did you know that spinner dolphins mate in orgies called wuzzles or that lionesses mate up to 157 times in 3 days when they’re in oestrous?) of the love and sex lives of animals, portrayed in intelligent and easy to read panels, videos (with the beautiful Isabella Rossellini) and taxidermied specimens. There were a few things that could have been different in my point of view though… Where were the playbacks of sexual callings that could have created a sexy tropical forest atmosphere? Why were the visitors’ paper notes’ wall constantly checked (censured?) for ‘unsuitable’ messages if the exhibition was primarily for adults?

But anyway, the exhibition is well worth a visit and clearly fulfilled its purpose and got me thinking about SEX (and a quote in one of their panels is acting like the title on this post). Just having a quick look through an array of scientific papers on the topic is enough to come across a variety of hypothesis on how sex could have originated and why it’s still maintained and spread across a huge number of species. Apparently (now hold on to the edges of your seats) sex is not entirely adaptive. Wait…what?! Yes, you read it right. There are two main problems with the understanding of the widespread sexual reproduction. First, because of recombination, sex has the great potential to destroy favourable gene combinations that would easily and most likely be preserved in the next generation if only it was generated but cloning of parentals. Second, sexual females will potentially only produce half the number of daughters than asexual females (‘cost of males’).

That’s fine, we get that, don’t we? But isn’t it a bit too simplistic? Maintenance of exact copies of adaptive sets of genes would sound perfect in an unchangeable world but looking at the history of Earth, there weren’t many times when the environment was kept entirely biotically or abiotically constant. And when in an changing environment variability is what provides the capability of adaptation to the new conditions and having that possibility to cope with still unknown variables must be worth having some less adaptive sets of genes in a population. After all, the truly maladaptive genes should be purged from the population pool by means of natural selection while the current more adaptive ones are maintained and spread in the following generations.

Of course I am definitely not the only one who thinks sex is actually very much adaptive and there are a bunch of theories around trying to demonstrate exactly how that is. One is the environmentally biased Red Queen hypothesis, by which sex is adaptive in a world full with pathogens, another is the mutational approach, by which recombination would prevent the loss of temporally bad alleles by mixing them with the temporally good ones, maintaining the variability and potential for future adaptation in a changing environment. Or even a more ‘pluralist approach’ by adding both mechanisms to the model. Hey, they make sense, right? World full with pathogens? CHECK! Need to maintain different alleles for potential future adaptation? CHECK again!

I myself am convinced of all of sex adaptive characteristics. Although asexual reproduction is clearly widespread and functioning quite well for unicelular organisms (prokariotes or eukariotes such as yeasts, or the protists with their facultative rounds of sex), multicelular organisms are not able to cope with the “supersonic” velocity by which bacteria (for example) reproduce and would need a much longer time frame to come up with a sufficient number of generations that would statistically be enough for the right mutations to occur if those mutations would be the only source of variability on which sexual selection could act. Do not get me wrong, mutations do occur and they are hugely responsible for the evolution of species but Darwin pinpointed the importance of variation within species and sex and recombination are key mechanisms in keeping that.

Now, it’s not only because sex is adaptive and necessary for species of animals and plants that it is less of a war. Focusing on animals, females are a limiting resource for males (or the other way around for species where there is sex role reversal) and reproduction has a much higher cost for females, who need to make sure they choose the best candidates for fathering their babies once that’s a commitment that results in them carrying and nurturing the offspring during their embryonic stages and very often for a long time after birth (species where there is parental care). It’s all about passing your genes to the next generation and making sure that the better ones do.

So males (again, just usually) fight between themselves in order to have access to females (see picture on the left for examples of weapons) or they try to show females how superior they are by their flashy colourful spots, feathers, big muscular bodies or the beautiful songs and callings they perform (did anyone say PEACOCK? Point for you). All these cost a lot of energy and all this happens in an environment that is normally constantly changing, with limiting food resources, crowded with pathogens and predators just waiting for their next meal. So only the best ones, only the ones with the most adaptive or at least adaptive enough get to survive and mate, passing their selected genes and making sure the species prevails.

Sex indeed feeds evolution. And I am glad it does for some of the most beautiful behaviours and morphologies have originated to please and woo our patners.

Quoting but inverting another sentence present at the panels of the NHM Sexual Nature exhibition, asexual reproduction might even be twice as efficient as sexual reproduction, but is not half as fun.

Suggested readings:

Birky Jr C W (1999). An even broader perspective on sex and recombination. Journal of Evolutionary Biology 12, 1013.

Crow, J F (1999). The omnipresent process of sex. Journal of Evolutionary Biology 12, 1023.

Futuyma D J (1998). The evolution of genetic systems In: Evolutionary Biology. Sinauer Associates: Massachusetts.

Otto S P and Nuismer S L (2004). Species interactions and the evolution of sex. Science 304, 1018.